1. Field of the Invention
This invention relates to the field of well logging and, more particularly, to a method of determining earth formation resistivity in real-time and at extended depths of investigation into the formation. A form of the invention has general application to the well logging art, but the invention is particularly useful in logging while drilling (LWD) and directional drilling applications.
2. Description of the Related Art
Resistivity logging, which measures the electrical resistivity of formations surrounding a borehole, is a commonly used technique for formation evaluation. Porous formations having high resistivity generally indicate the presence of hydrocarbons, while other porous formations having low resistivity are generally water saturated. Wireline well logging makes resistivity measurements in a borehole (with the drill string removed) by lowering a logging device into the borehole on a wireline cable and taking measurements with the device while withdrawing the cable. This resistivity measurement technique uses various arrangements of sensors and detectors, on the logging device and at the earth's surface, to measure electrical currents and/or potentials from which one derives formation resistivity.
During the directional drilling of a well in an earth formation it is necessary to determine the trajectory of the well in order to ensure that the well is being drilled in the desired direction. To accomplish the task of determining the well trajectory drillers take several measurements of the drill bit and borehole conditions during the drilling process. These downhole measurements include the inclination and direction of the borehole near the bit, which are essential for maintaining accurate control over borehole trajectory. During the drilling process knowledge of formation properties can be useful in connection with borehole trajectory control. For example, identification of a "marker" formation, such as a layer of shale having properties that are known from logs of previously drilled wells and which is known to lie a certain distance above the target formation, can be used to great advantage in selecting where to begin curving the borehole to ensure that a certain radius of curvature will indeed place the borehole within the targeted formation. A shale formation, for example, can generally be detected by its relatively high level of natural radioactivity, while a sandstone formation having a high salt water saturation can be detected by its relatively low electrical resistivity. Once the borehole has been curved so that it extends generally parallel to the bed within the target formation, these same measurements can be used to determine whether the borehole inclination in the target formation is too high or too low.
The focus of the present invention is in the area of measuring formation resistivity during drilling operations. Resistivity measurements typically involve one of several techniques. The first of these techniques uses a system of toroids and electrodes. An electrical current is generated at a toroidal transmitter and passes into the formation. The current travels through the formation and an electrode positioned at a distance away from the transmitter detects the current or voltage drop. The formation resistivity is derived from the current and/or voltage measurement. This electrode resistivity measurement technique is described in U.S. Pat. Nos. 5,235,285; 5,339,036; 5,339,037; and 5,359,324, which are incorporated herein by reference.
A second technique for measuring resistivity is an electromagnetic wave propagation measurement which measures the phase shift and/or attenuation of a signal between a pair of receivers. Examples of this technique are described in U.S. Pat. No. 4,899,112 and 5,594,343, which are incorporated herein by reference.
A third type of resistivity measurement is the induction technique. This technique utilizes a system of coils wrapped around a metallic or non-metallic mandrel and is described in U.S. Pat. No. 5,157,605, which is incorporated herein by reference. With the induction technique, the signal at the receiver is proportional to the conductivity of the formation. The signal is generated by inducing currents in the formation and detecting voltage at the receiver.
These techniques are commonly used to determine formation resistivity with maximum radial depth of investigation roughly equal to the maximum transmitter to receiver distance in the logging apparatus. The maximum depth of investigation for typical wireline logging and LWD systems is therefore generally limited to about 6 to 8 feet (1.8 to 2.4 meters) due to cost-driven and practical limitations on tool length.
Referring again to the induction technique, induction tools employ alternating currents in transmitter coils to set up an alternating magnetic field in the surrounding conductive earth formation. This changing magnetic field induces current loops in the earth formation which themselves produce a secondary magnetic field that is detectable as a voltage by a receiver coil placed at a distance from the transmitter coil.
Generally, induction tools consist of multi-coil arrays designed to optimize vertical resolution and depth of investigation. FIGS. 1 and 2 schematically illustrate a basic two-coil wireline induction tool 8 deployed in a borehole 9. A two-coil tool comprises a transmitter coil 1 and receiver coil 2 mounted coaxially on a mandrel 3. Typical coil separations range from 1 to 10 feet (0.3 to 3.0 meters). In practice, each coil may consist of from several to a hundred or more turns, with the exact number of turns determined by design considerations. A transmitter oscillator 4 controls the operating frequency of the induction tool 8 which is generally in the tens of kiloHertz (kHz) range, with 20 kHz being the most commonly used frequency. The transmitter coil 1 induces a current 5 in the earth formation 10 which is detected by the receiver coil 2. This current forms a ground loop 6 around the tool. Receiver amplifier 7 amplifies the received signal, from the secondary magnetic field generated by the sum of all the ground loops in the formation, for processing and further transmission uphole.
In spite of the fact that induction is referred to as a "resistivity" measurement, the voltage induced in a receiver coil, with the direct mutual signal removed by design, is actually directly proportional to the earth formation conductivity rather than to the earth formation resistivity. Contributions to the total conductivity signal from various individual regions of the formation sum electrically in parallel, because the currents generated by the coaxial coil arrangement all flow parallel to one another.
In addition to measuring formation resistivity, electrode and induction techniques have other applications associated with logging while drilling operations. One such application is downhole telemetry, which uses these techniques in the transmission of measurement data uphole to a receiver and eventually to the surface for interpretation and analysis. Electrode telemetry is implemented by means of a transmitting toroidal coil antenna disposed on the tool body which is energized to induce a current representative of measured data which current travels in a path that includes the tool body and the earth formation. The tool also has an electrode disposed on the body a distance from the transmitter. The electrode detects an electrical signal resulting from the induced current and obtains the measurement data from the detected current. This telemetry system is disclosed in further detail in U.S. Pat. No. 5,235,285.
Effective downhole electrode telemetry must overcome several obstacles caused by the electrical characteristics of the borehole and formation. During drilling and logging operations drilling fluid passes through the tool to the drill bit. Drilling fluid also fills the borehole annulus between the logging tool and the borehole wall. If this drilling fluid is oil-based and consequently electrically resistive, it will affect the signal strength during transmission. Still another obstacle can be a very resistive formation or very conductive thin layers embedded in a resistive formation. These types of layers are especially troublesome during telemetry operations. Very resistive formations severely restrict the flow of current. This restriction of current is analogous to an electrical open circuit. Where conductive formation layers are embedded in a resistive formation the flow of current to the receiver is prevented by the conductive layers acting as a short circuit or creating a current pinching effect. Another obstacle to transmission of signals uphole is the use of equipment, such as stabilizers, on the drill collars between the transmitter and receiver. Such equipment can act as an electrical short which prevents the telemetry signal from reaching the receiver.
Downhole induction telemetry overcomes several of the obstacles encountered by electrode telemetry. A typical induction telemetry system comprises a transmitting antenna and a modulator positioned at a first location downhole. A signal modulated to carry data acquired by one or more measurement sensors is applied to the transmitting antenna to induce a magnetic field about the location. A receiving antenna, positioned at a second location uphole, intercepts a portion of the magnetic field induced by the transmitting antenna and produces a signal that is demodulated to yield the transmitted data.
The present invention comprises a method and apparatus for providing a real-time deep resistivity measurement of earth formations with a depth of investigation ranging from approximately 25 to 60 feet (7.6 to 18.3 meters) from the measuring instrument. The present invention also describes an apparatus and method providing an improved telemetry system for sending measurement data uphole. The resistivity measurement is derived from the induction telemetry signal transmitted uphole.
It is therefore an object of the invention to measure formation resistivity at relatively deep radial depths of investigation from the measuring device.
It is another object of the invention to effect a resistivity measurement using the telemetry signal that carries data of measured parameters uphole to a receiver.
It is another object of the invention to detect formation boundaries while drilling, especially in directional drilling applications.
It is a further object of this invention to provide for more effective formation resistivity measurements and data transmission in the presence of electrically resistive borehole fluids.